1. Field of the Invention
The present invention is in the field of storage bins for solid particulate materials, such as grain. More particularly, there is described a bin that includes a number of modules of similar shape but increasing size which are connected in a sequence. The resulting bin will exhibit mass flow with less vertical headroom required than in existing designs, especially when friction angles are high.
2. The Prior Art
Several considerations drive the design of hoppers. First, it is important that the material not form a bridge or arch within the hopper, because an arch interferes with or terminates the flow of material from the bottom of the hopper. If and when the arch collapses, the material may surge from the hopper. It is well known that arcing can be eliminated if the opening at the bottom of the hopper is large enough. For a right circular conical hopper, the critical gravity flow arching dimension for a particular material is designated as B.sub.c. As will be seen below, some embodiments of the present invention permit the use of discharge openings that are only a fraction of B.sub.c.
A second consideration in the design of hoppers is that the wall of the hopper must be steep enough so that the material will slide smoothly along the wall during discharge. If the wall is not steep enough, a thick layer of the material will cling to the wall and discharge will take place from only a limited region near the axis of the hopper, a condition referred to as "rat-holing." For a hopper having the shape of a section of a right circular cone, the largest semi-apex angle at which mass flow will occur, for a particular material, is denoted by .theta..sub.c, the mass flow angle for that particular material. As will be seen below, the present invention permits the use of semi-apex angles that are appreciably greater than .theta..sub.c.
A further consideration in the design of hoppers is the optimization of the geometry of the hopper within the constraints described above. Normally, in most applications one would prefer, for a given volume, the hopper which is shortest in height. From elementary geometry it is known that the volume within a truncated right circular cone is given by the relation ##EQU1## where d is the diameter of the smaller end, where H is the height, and where .theta. is the semi-apex angle of the truncated cone. The dependence of the volume on the semi-apex angle .theta. is very strong. For example, for a typical hopper with d=1 and H=5 the volume will increase by a factor of 1.97 as the angle .theta. increases from 20 degrees to 30 degrees. This effect is even more pronounced for smaller values of .theta. such as would be required for materials that are more cohesive. For example, for the same typical hopper, the volume increases by a factor of 2.38 as the semi-apex angle .theta. increases from 10 degrees to 20 degrees. As will be seen below, the present invention permits the use of semi-apex angles appreciably greater than .theta..sub.c, and for a given volume this results in a bin having considerably less height.
Although conical, rectangular and chisel-shaped hoppers are known in the art, hoppers having the unique shape described herein are believed to be new and advantageous.
The following technical articles by the present inventor show the state of the art: "Design for Flexibility in Storage and Reclaim," Chemical Engineering, Oct. 30, 1978, pp. 19-26; "Selection and Application Factors for Storage Bins for Bulk Solids," Plant Engineering, July 8, 1976; Stress and Velocity Fields in the Gravity Flow of Bulk Solids, Journal of Applied Mechanics, 1964, Series E 31 pp. 499-506; "Feeding," Chemical Engineering, Oct. 13, 1969, pp. 75-83 "Method of Calculating Rate of Discharge from Hoppers and Bins," Transactions of SME, Mar. 1965, Vol. 232, pp. 69-80; and "New Design Criteria for Hoppers and Bins," Iron and Steel Engineer, Oct. 1964, pp. 85-104 (with Colijn, H.).